1. Field of the Invention
The present invention relates to a control system for a continuously variable transmission of a motorcycle, and more particularly to a control system for a continuously variable transmission of a motorcycle, which is equipped with a centrifugal clutch.
2. Description of the Related Art
Conventionally, a motorcycle, such as a scooter, is known in which output from an engine is transmitted to a driving wheel via a continuously variable transmission (CVT) and a centrifugal clutch. A control system for the CVT changes the diameter of a drive pulley, around which a V-belt is stretched, by electronic control to thereby continuously change the transmission ratio, i.e. a ratio between the rotational speed of the engine and that of a drive shaft. More specifically, by searching a transmission map according to the degree of opening of a throttle valve and the speed of the motorcycle, the diameter of the drive pulley is continuously changed to thereby set the transmission ratio to a desired value.
The centrifugal clutch is disposed at a location downstream of the CVT, more specifically, between the CVT and the drive shaft so as to allow a rider to handle the motorcycle with ease when the engine is in stoppage.
The idling speed of the engine is set to a different value depending upon whether the engine is in an idle-up state at the start of the engine or in a warmed-up state after the lapse of a predetermined time period from the start of the engine, in which the engine rotational speed has become stable. In the idle-up state during cold start of the engine, the idling speed is set to a highest value of e.g. 1450 rpm, whereas in the warmed-up state, the idling speed is set to a lowest value of e.g. 1200 rpm.
The centrifugal clutch is configured such that the engine is disconnected from the drive shaft when the idling speed is at the highest value in the idle-up state during cold start. An engine rotational speed (disconnecting rotational speed) at which the engine is disconnected from or connected to the drive shaft is set to a value slightly higher than the highest idling speed, and the disengagement of the centrifugal clutch for disconnecting the engine from the drive shaft is carried out at this engine rotational speed (disconnecting rotational speed).
For this reason, when the engine is being driven at the highest idling speed in the idle-up state, the centrifugal clutch is required to be rotating at a rotational speed not higher than the disconnecting rotational speed. The rotational speed of the centrifugal clutch is increased as the transmission ratio becomes lower, i.e. changed to a higher speed ratio, and therefore, the CVT has to be set to a transmission ratio (disconnecting transmission ratio) not lower than a predetermined transmission ratio at which the centrifugal clutch rotates at the disconnecting rotational speed. To this end, in the above-mentioned transmission map, the transmission ratio is set such that the rotational speed of the centrifugal clutch becomes not higher than the disconnecting rotational speed in a low-speed range of the motorcycle (see the publication of Japanese Patent No. 3194641, for example).
Accordingly, in the warmed-up use where the idling speed is lower than in the idle-up state, for a low-speed range of the motorcycle where the engine is being driven in the vicinity of the idling speed, the transmission map is configured such that the CVT is set to a high transmission ratio, i.e. a low-speed ratio.
On the other hand, in a low-speed range of the motorcycle immediately before stoppage of the motorcycle, i.e. in the low speed range where the engine rotational speed is close to the idling speed, the steerability of the vehicle is better when engine braking is not very effective, and therefore, it is preferred that the centrifugal clutch is disengaged, or the transmission ratio of the CVT is set to a higher speed ratio.
However, when the motorcycle is traveling in a low-speed range, to disengage the centrifugal clutch, it is required to set the transmission ratio of the CVT to a lower speed ratio, and when the transmission ratio is set to such a lower speed ratio, engine braking becomes more effective, which causes further degradation of the steerability of the motorcycle. Further, if the transmission map is configured such that the transmission ratio of the CVT is set to a higher speed ratio when the motorcycle is traveling in the low-speed range in which the engine rotational speed is close to the idling speed, the rotational speed of the centrifugal clutch is increased in this speed range. Therefore, when the engine rotational speed becomes equal to the idling speed in the idle-up state (the highest idling speed as mentioned above), the rotational speed of the centrifugal clutch becomes higher than the disconnecting rotational speed. Therefore, in this speed range the transmission ratio should not be set to such a higher speed ratio.